Master Chief wages war not only against the Covenant but also against the Flood, which was frozen in the depths of the Halo ring and accidentally awakened by the Covenant and humans. Presumably, the Flood caused the demise of the Forerunners – an ancient race that seems very similar to humans; they built all these Halo installations. At the moment when almost the entire Forerunner race was destroyed by the Flood, the rings were activated, which destroyed all life in the area. But the High Hierarchs of the Covenant, the so-called Prophets, use the rings as a key to great purification and salvation, and they call the Covenant (an agreement that includes six alien races) the sword in the hand of the true god. Their god, Halo, is this ring-shaped structure.